This invention relates to a process, and to molding apparatus, for the production of composite metal articles comprising bimetallic plate.
Numerous prior art proposals for producing composite metal articles are discussed in U.S. Pat. No. 4,953,612 to Sare et al (filed as PCT/AU84/00123). Those proposals suffer from various disadvantages or limitations, at least some of which are overcome by the teaching of U.S. Pat. No. 4,953,612. The teaching of U.S. Pat. No. 4,953,612 is well suited for the manufacture of a range of composite metal articles comprising a cast component bonded to a substrate component. However, the teaching is less well suited for the production of a composite metal article comprising bimetallic plate, in particular plate which is relatively thin and/or has a relatively large surface area. Thus, the teaching of U.S. Pat. No. 4,953,612 can encounter difficulties, such as uneven bonding, in the production of bimetallic plate in sizes greater than about 300xc3x97300 mm, with a thickness of less than about 30 mm and a thickness ratio of about 1:1 or less for cast metal to substrate.
The present invention seeks to provide a process and molding apparatus which enables production of relatively large area, bimetallic plate, such as up to and in excess of 1800xc3x971500 mm, while indications are that plate at least up to 3000xc3x971650 mm is able to be produced.
In the process of the present invention a plate (hereinafter referred to as a xe2x80x9csubstratexe2x80x9d), which is formed of a first metal, has a component (hereinafter referred to as xe2x80x9ccladdingxe2x80x9d) of a second metal cast against it to form bimetallic plate. The first metal for the substrate may be titanium, nickel or cobalt, a ferrous alloy or a titanium-, nickel- or cobalt-base alloy. The second metal for the cladding may be copper, nickel or cobalt, a ferrous alloy or a copper-, nickel- or cobalt-base alloy. While not necessarily the case, the first and second metals usually are compositionally different. However, where the first and second metals are the same or similar, in being closely related compositionally, this can be to achieve a difference in properties based on microstructure, such as due to the substrate being hot- or cold-worked and the cladding having an as cast microstructure.
As in U.S. Pat. No. 4,953,612, the surface of the substrate against which molten alloy is to be cast to form the cladding needs to be rendered substantially oxide-free. Also, the substrate is preheated and is protected against oxidation by a suitable coating. The coating may be formed from flux which is applied over the substrate surface, and melted to form a protective film during preheating. However, other protective coatings can be used, such as a deposit of a suitable metal formed for example by electroless or electrolytic plating of nickel or another metal, or a non-metallic coating such as of colloidal graphite containing a silicate binder. Depending on the protective coating use, it is either displaced by or alloyed with the alloy cast to form the cladding, facilitating wetting of the substrate surface by the cast alloy.
Also as in U.S. Pat. No. 4,953,612, the molten alloy to form the cladding is poured at a superheated temperature to facilitate the attainment, with preheating of the substrate, of an overall heat energy balance to achieve a diffusion bonding between the cladding and the substrate. The diffusion bond is obtained substantially in the absence of fusion of the substrate surface against which the cladding is cast.
In the production of bimetallic plate, it can be very difficult to achieve a sufficient heat energy balance for good bonding between the cladding and substrate. This is particularly the case where the plate is large in area, and/or relatively thin and/or has a relatively low thickness ratio of cladding to substrate. Under these conditions, it is found that loss of heat energy to the mold becomes a significant factor preventing the attainment of such energy balance, with this loss being from both the preheated substrate and from the molten alloy as it flows over the substrate. This loss can be exacerbated by delays between preheating the substrate and pouring the molten alloy to provide the cladding and/or by an unduly long period during which the molten alloy is poured. Also, it is found that loss of uniformity of heat energy balance, with resultant non-uniformity of bonding, can result from uncontrolled or irregular flow of molten alloy over the substrate, such as to give rise to an unduly long flow path and/or a reducing flow rate for the alloy.
We have found that substantially improved bimetallic plate can be produced by controlled casting of molten, alloy to provide the cladding. In the process of the invention, the cast alloy is caused to flow across the surface of the substrate along a controlled melt front which is advanced in a manner which, having regard to the temperature to which the substrate is preheated and the superheat temperature of the molten alloy, provides over substantially the entire surface of the substrate a heat energy balance within limits sufficient for achieving a diffusion bond between the cladding and substrate.
While not necessarily the case, the bimetallic plate may be square or other rectangular form. For ease of further description, a rectangular substrate and resultant plate is assumed in the following. Also for ease of description, directions across the substrate are designated as longitudinal, for the direction in which the melt front advances, and lateral for the direction in which the melt front extends transversely with respect to its direction of advance. However, while the substrate and resultant plate may have a longitudinal extent which is greater than its lateral extent, the converse may apply or the longitudinal and lateral extents may be substantially equal. Additionally, while the longitudinal direction of melt front advance can be substantially between longitudinally opposite edges of the substrate, longitudinal melt advance can be over part of the longitudinal extent of the substrate. Moreover, the lateral extent of the melt front and, hence, the width of cladding in that direction, may be over substantially the full lateral extent of the substrate or over a part of that extent.
In the process of the present invention, a controlled melt front is advanced in a manner providing required heat energy balance for bonding by at least one of the following features:
(a) causing the molten alloy to enter a mold cavity, in which the substrate is positioned, through a laterally disposed series of gates providing communication between a runner and the mold cavity, whereby the molten alloy forms a laterally extending melt front, and
(b) causing the melt front to advance longitudinally over the substrate at a rate which is substantially uniform across the lateral extent of the melt front.
The process of the invention preferably utilizes each of features (a) and (b).
Thus, according to the present invention, there is provided a process for the production of a composite bimetallic plate, wherein the process comprises the steps of:
(a) rendering a major surface of a substrate plate formed of a first metal substantially oxide-free;
(b) providing a suitable coating over said oxide-free major surface whereby said major surface is protected against oxidation;
(c) preheating the substrate plate to a sufficient temperature;
(d) positioning the substrate plate in a mold cavity of a mold with said major surface facing upwardly and substantially horizontally to thereby fill a lower portion of the depth of the mold cavity;
(e) securing the substrate plate in the mold cavity; and
(f) casting a cladding of a second metal over said major surface of the substrate plate to form, with the substrate plate, said bimetallic plate wherein said cladding is cast by pouring, at a sufficient superheated temperature, a melt of the second metal for flow of the melt into the mold cavity to fill an upper portion of the depth of the mold cavity,
wherein the securing step (e) secures the substrate plate whereby the substrate plate is substantially restrained against buckling during the casting step (f), and wherein the temperature to which the substrate plate is preheated in step (c) and the superheated temperature of step (f) achieve an overall heat energy balance between the first and second metals whereby a diffusion bond substantially free of fusion of the major surface of the substrate plate is achieved therebetween on solidification of the melt;
and wherein the process further comprises the steps of:
(g) causing the melt poured in step (f):
(i) to flow in at least one elongate runner which extends along a first edge of the substrate plate, and
(ii) to enter the mold cavity through a series of gates providing communication between the runner and the mold cavity along said first edge of the substrate plate,
whereby the melt is at substantially the same pressure at each gate and on entering the mold cavity forms a laterally extending melt front along said first edge of the substrate plate; and
(h) causing the melt to fill the upper portion of the mold by said melt front advancing over said major surface away from said first edge at a rate which is substantially uniform across the lateral extent of the melt front, whereby attainment of the required heat energy balance is facilitated.
The invention also provides a molding apparatus, for use in producing composite bimetallic plate comprising:
a mold having a drag section and a cope section which together define a mold cavity having a form substantially corresponding to bimetallic plate to be produced therein;
at least one elongate runner defined by the mold and extending along a first end of the mold cavity; and
a series of laterally spaced gates which are defined by the drag and cope sections of the mold and which provide communication between the at least one runner and the mold cavity at said first end;
wherein a lower portion of the mold cavity is defined by the drag section of the mold and has a substantially flat, substantially horizontal support surface which extends between said first end and a second end of the mold cavity remote from the first end, and on which a substrate metal plate is positionable whereby a major surface of the plate faces upwardly and is substantially horizontal; and
wherein the apparatus further comprises means for securing a substrate positioned on said support surface and thereby restraining the substrate plate against buckling during casting of cladding thereon.
To enable attainment of feature (a), molding apparatus according to the invention includes a mold defining a mold cavity in which a substrate is positionable, and in which molten alloy is able to be cast against an upper surface of the substrate. The mold defines at least one feed sprue by which molten metal is receivable, with the feed sprue communicating with at least one lateral runner by which molten metal passes from the feed sprue to each gate of the series. At least where the cladding is to extend from a transverse edge of the upper surface of the substrate which is adjacent to the series of gates, the mold cavity may have a galley portion at which the gates communicate with the cavity.
In a casting operation with a mold providing for feature (a) molten metal flows into the mold cavity via each gate with streams of molten metal from successive gates merging to generate a molten metal melt front which passes longitudinally over the upper surface of the substrate. Where the mold cavity has a galley portion, the merging of streams preferably occurs in the galley portion before the melt front reaches the substrate.
To enable attainment of feature (b), the lateral runner may be configured substantially to equalize metal pressure at each gate of the series. For this purpose, the runner can decrease in cross-section after each successive gate in a direction extending laterally away from the feed sprue, such as by the runner having stepwise reductions in its depth. Additionally, or alternatively, attainment of feature (b) can be facilitated by the mold being configured so that the substrate, when positioned in the mold cavity, has its upper surface inclined upwardly from the feed sprue, i.e. inclined upwardly in the direction of melt front advance. Thus, across its lateral extent, the melt front is constrained to a substantially uniform advance, under the influence of gravity.
While it usually is preferred for the substrate to have its upper surface substantially horizontal or inclined upwardly from the feed sprue, there can be benefit in having the surface slightly inclined downwardly from the sprue. That is, the upper surface may be inclined downwardly in the direction of melt front advance. The downward inclination has the benefit of increasing the flow velocity of the metal. The extent to which the inclination is possible is dependent upon melt viscosity, and the magnitude of the inclination needs to be limited so as to ensure that a substantially uniform rate of melt front advance is maintained across the lateral extent of the front.
Sand molds have been found to be well suited for use in the present invention, although a castable refractory material can be used instead of sand to form the molds. The mold is designed to separate in two main sections, namely a drag section and a cope section. The drag and the cope sections preferably are contained in steel mold support frames by which the mold sections can be clamped together, such as mechanically or hydraulically. The drag section has a cavity in which the substrate is positionable and which forms at least part of the mold cavity. The drag section may have a sprue well into which molten alloy is received from the feed sprue, while it also may have at least one lateral runner. The cope section has the bottom part of the feed sprue, while it may have a cavity which forms part of the mold cavity and in which the cladding is cast. The cope section also may have the lateral series of gates and remote from the feed sprue bottom part and the gates, the cope section may have a lateral cavity for receiving excess cladding alloy.
The mold sections preferably are able to be clamped together with a clamping force which, in combination with the mold design, ensures adequate mold sealing and adequate restraint on the substrate edges during the cladding operation is able to be achieved. Thus, recourse to sealing aids provided between opposed or mating surfaces of the mold sections can be avoided, with a saving in time between preheating the substrate and closing the mold in preparation for casting cladding alloy.
In one suitable arrangement, the draft and cope sections of the mold are made, in their respective support frames, from a molding sand and a binder, such as a sodium silicate binder or an organic binder. A silica sand is suitable, although other molding sands such as olivine or zircon sands can be used. To reduce erosion by molten alloy, critical areas of the runner and gating system may be molded from bonded sand, such as silicate bonded sand selected from olivine, zircon or chromite sand or, if molded from silica sand, those areas can be protected by refractory mold paint. Also, to improve the surface finish of the cast cladding, the mold cavity surface of the cope section may be coated with a refractory mold paint. The support frame for each section may be constructed from fully welded mild steel channel sections, preferably with the drag section frame including a steel bar passing underneath the sprue well to support the sand against the force of poured molten alloy.
In the mold of that arrangement, the dimensions of the cavity in the drag section, particularly in the lateral and longitudinal directions, are sufficient to allow for thermal expansion of the substrate. However, when the substrate is positioned in that cavity, its upper surface preferably is flush with an opposed, peripheral, upper surface of the drag section by which the latter is engaged by a peripheral, lower surface of the cope section. The cope section, when clamped to the drag section, preferably acts to provide a clamping action on margins of the substrate, such as detailed later herein.
As indicated, the substrate is preheated prior to the casting of cladding alloy. It is highly desirable that there be minimum delay between the completion of preheating and the commencement of casting, while preheating the substrate after it is positioned in the drag section cavity is the most practical option. In practice, it is not possible to completely uniformly preheat the substrate and, as a result, the substrate deforms or buckles, usually by a central region bowing upwardly but with some lifting at edges also being likely. Casting of cladding alloy with the substrate in this form exacerbates deformation or buckling and further makes difficult the production of useful bimetallic plate. Also, the deformation or buckling can be such as to make difficult the attainment of feature (b) detailed above. Thus, the deformation or buckling of the substrate therefore needs to be minimized or obviated,
Threaded metal studs welded to the lower surface of the substrate and restrained by nuts tightened against the drag mold frame can be used to offset or prevent deformation or buckling of the substrate. The deformation or buckling alternatively can be offset by utilizing the force by which the drag and cope sections of the mold are clamped together, so as to generate compressive loads acting to press the substrate to an approximately flat condition. In one suitable procedure for this, a series of laterally spaced, longitudinally extending metal strips are tack-welded to the upper surface of the substrate, thus forming longitudinal channels on the substrate along which the cast alloy is able to flow. In still another suitable procedure, a plurality of metal chaplets are tack-welded to the upper surface of the substrate in a suitably disposed array. The metal strips, which are dimensioned to form channels of a depth corresponding substantially to the required cladding thickness, may be of a similar composition to the cast alloy and become incorporated therein as part of the cladding. The chaplets, which have a thickness corresponding substantially to the required cladding thickness, also may be of similar composition and become incorporated in the cladding.
On closing the mold and clamping the drag and cope sections together, the clamping force causes the cope section to engage the strips or chaplets with generated compressive forces thereby acting to force the substrate down against the drag section. The substrate can be forced into a somewhat flat condition, but with minor bowing between successive strips or chaplets. The compressive forces are such that the substrate is able to be retained substantially in that condition during casting of the cladding.
The use of longitudinal strips or of chaplets in a central region of the substrate, to achieve such somewhat flattened condition, results in edges of the substrate being urged downwardly in the drag section cavity. Due to this, molten alloy for forming the cladding can be substantially prevented from flowing under the substrate. However, it can be beneficial to positively hold down the substrate at longitudinal side edges. For this latter purpose, a respective longitudinal refractory bar, for each of those edges of the substrate, may be molded into the cope section of the mold at a location at which it engages and holds down an edge of the substrate when the drag and cope sections are clamped together. Alternatively, where the sand of the cope section has sufficient strength, it can overlap and hold longitudinal edges of the substrate when the drag and cope sections are clamped together.
Where the mold sections abut at opposed peripheral surfaces as they are clamped together, the area of contact is sufficient to enable the sand of the mold sections to withstand the clamping force. Also, an area of cope sand directly over each lateral edge of the substrate, such as by 25 to 30 mm, can withstand compressive forces exerted on it by the bending forces generated in the substrate edges due to thermal stresses. However, at longitudinal strips or at chaplets used to flatten the substrate, the compressive forces per unit area can reach a level at which damage to the sand of the cope section can occur. To avoid this, the cope section can include ceramic pins, ceramic-tipped metal pins, longitudinal refractory bars or the like which transfer the compressive forces to the strips or chaplets. The pins, bars or the like may be fixed to or engaged with the support frame of the cope section, such that the compressive forces are transferred from the cope section support frame, to the substrate, via the pins, bars or the like and via the strips or chaplets.
Immediately adjacent to the gates, there can be difficulty in holding down the adjacent lateral edge of the substrate. Consequently, there is a risk of that edge of the substrate lifting during casting, and molten metal penetrating under the substrate. This risk is high due to thermal gradients from the upper to the lower surface of the substrate, caused by the superheated molten metal and its fast flow rate and the resulting bending forces in the substrate. However, if chaplets are used to hold down the lateral edge of the substrate adjacent to the gates they are likely to be dissolved rapidly by the fast flowing molten metal unless they are of a sufficient size and/or placed outside the direct metal stream emanating from the gates. A similar situation can occur if, rather than use of chaplets, longitudinal metal strips are used to hold down the substrate unless the strips are positioned out of direct alignment with any of the gates so that little or no turbulence is created in the metal flow and there is little chance of the strips dissolving too quickly. Accordingly, an alternative way is desirable to offset deformation or buckling of the substrate resulting in lifting of its lateral edge adjacent to the gates.
One suitable way in which to restrain lifting of the lateral edge of the substrate is to bend the substrate so as to cause the lateral edge to be forced down onto the drag mold sand. Another suitable way to restrain the lateral edge is to weld a strip of steel to the underside of substrate along that edge. A suitable strip, such as of mild steel, may for example be about 25xc3x976 mm in cross-section and welded on edge for a substrate of about 10 mm thick. The strip is accommodated in a correspondingly positioned lateral groove in the drag section at which the depth of the drag section cavity is increased. During casting, location of the strip in that groove prevents penetration of molten alloy beneath the edge of the substrate.
For use in the present invention, there may be a casting station providing solid support for the drag section of the mold, means for convenient manipulation of a preheat furnace, and means for accurate placement and clamping of the cope section in relation to the drag section on completion of a preheat cycle for a substrate. At the casting station, there may be a support structure mounted on a solid support surface, with the drag section resting on or secured to the support structure by its frame. Adjacent to the support structure, there is means for pouring molten alloy for casting the cladding. This may be a ladle into which the alloy is received from a nearby furnace. However, it is preferred that the furnace is adjacent to the support structure and is adapted for pouring the molten alloy into the mold. The furnace may for example be an induction tilt furnace.
The cope section of the mold may be supported or mounted so as to be able to be raised from and lowered to a position in which it is able to be clamped to the drag section, as required. This movement of the cope section may be by any suitable device, such as by an overhead hoist, extendible hydraulic actuators or the like. The frame of the cope section preferably is provided with rollers which ride on posts of the support structure and thereby guide the cope section in its movement.
In its raised position, the cope section may be spaced above the drag section sufficiently to enable the preheat furnace to be positioned therebetween. The support structure may include horizontally disposed rails along which a carriage, which forms part of or supports the preheat furnace, is able to travel between a retracted position, and an advanced position in which the preheat furnace is above the drag section.
The preheat furnace can take a variety of forms, such as a gas burning preheater, an induction preheater or an electric element preheater. For trials with 10 mm thick substrates about 1950 mm long and 1050 mm wide, one form of suitable preheat furnace had a downwardly open stainless steel shell with 125 mm thick low heat capacity insulation to the internal top and side surfaces, and helical nichrome alloy wire elements supported by ceramic tubes. This furnace was connected to a three phase 415V control box and had a maximum power output of 150 kW.